The invention relates to an electric incandescent lamp comprising:
a tubular lamp vessel which is closed in a vacuumtight manner, which is made of glass having an SiO.sub.2 content of at least 95% by weight, and which has an axis and seals on said axis;
a coiled tungsten incandescent body having turns and end portions, axially arranged in the lamp vessel;
support means between the end portions keeping the incandescent body centered in the lamp vessel and incandescent turns of that body spaced from the lamp vessel;
current conductors which enter the lamp vessel through respective seals and which are connected to respective end portions of the incandescent body.
Such an electric incandescent lamp is known from U.S. Pat. No. 5,523,650.
The known lamp is an IR radiator whose incandescent body is surrounded by quartz glass which comprises samarium.sup.III oxide and aluminum oxide so as to color it red. The lamp vessel may be linear or may be bent into a polygon which is open at one side. The incandescent body is helically coiled. Between the end portions several pieces of tungsten wire are present which are wound onto the incandescent body and which spiral to rest against the lamp vessel to act as support means in order to keep the incandescent body spaced from the lamp vessel. The reason why the support means separate the incandescent turns is that quartz glass is not capable of withstanding the temperature of the incandescent turns during operation when touched by these turns.
Such an IR lamp is known from EP-B-0 560 420, wherein a precursor in the quartz glass is converted into a red pigment by means of a heat treatment during the final manufacturing stages of the lamp.
From U.S. Pat. No. 5,565,734 an electric incandescent lamp of the kind described in the opening paragraph is known in which the support means consist of a refractory metal sleeve, short-circuiting turns of the incandescent body and of a constriction made in the lamp vessel and contacting the metal sleeve. As a result of the short-circuiting, these turns are relatively cold and do not incandesce.
U.S. Pat. No. 5,556,191 discloses such a lamp in which the tubular lamp vessel is accommodated in a mirror-coated outer envelope.
An IR radiator is known from U.S. Pat. No. 4,857,709 wherein the incandescent body is helically coiled with turns which are so large that the incandescent body rests against the wall of the lamp vessel substantially circumferentially. In that case the incandescent body must be designed so as to have only a comparatively low temperature during operation which the quartz glass is capable of withstanding. A disadvantage of this is, however, that the lamp can only be used in those applications where IR radiation of comparatively great wavelength is required as radiated by the incandescent body of comparatively low temperature. The large dimension of the turns of the incandescent body furthermore render it difficult to introduce the incandescent body into the lamp vessel.
These are also disadvantages inherent in the lamp known from JP-A-1-227.377. Here a bent quartz glass, for example colorless transparent lamp vessel contains an incandescent body which was coiled around a mandrel of unround cross-section and whose turns lie against the tubular wall of the lamp vessel. The turns have partly relaxed their stress after manufacture, so that the incandescent body, when viewed axially, shows a star shape and entirely or substantially entirely occupies the lamp vessel in cross-sections thereof.
IR radiators, but also floodlighting lamps, have the characteristic that they consume comparatively high powers, from a few hundreds to a few thousands of watts. The lamps and the incandescent bodies are comparatively long for this reason. Coiling a helical incandescent body around a round core, a mandrel or needle, is comparatively expensive. In the case of coiling around a continuous mandrel, this mandrel, for example made of molybdenum, must be removed after the coiling process in that by dissolving. It is accordingly advantageous to coil around a comparatively short needle, a portion having the length of one incandescent body being cut off periodically once it has been moved off the needle. It is necessary during this, however, to hold on to the turns on the needle so that the needle can still draw on uncoiled wire so as to coil it. If turns already made were not retained on the needle, in fact, they would relieve their stress on the round needle and lose their clamping force around the needle. The necessity of preventing this renders the coiling machine not only expensive but also comparatively slow. Such a method of manufacturing incandescent bodies is known, for example, from U.S. Pat. No. 4,616,682. In proportion as the incandescent body is longer, the machine costs of coiling, or the cost of the coiling mandrel, its dissolving after coiling, and the recovery of the mandrel material from the resulting solution will be higher.